According to the state of the art, rigid polyurethane foams are produced from polyols with, on the average, three hydroxyl groups per molecule, at least difunctional isocyanates, catalysts, blowing agents and polysiloxane-polyoxyalkylene block copolymers as foam stabilizers, as well as, optionally, conventional additives.
A summary of the raw materials, which may be used, and of the methods, which may be employed, can be found in Ullmann's "Enzyklopaedie der technischen Chemie" (Encyclopedia of Chemical Engineering), 1980, volume 19, pages 301 to 341, and "Kunststoffhandbuch" (Plastics Handbook), volume VII, Polyurethanes, by G. Oertel, Carl Hanser Verlag, Munich, 1983, pages 246 to 331.
In general, polyetherols or polyesterols with, on the average, at least three hydroxyl groups per molecule are used, the OH number of the polyols generally being between 100 and 800.
As blowing agents, halogenated hydrocarbons with boiling points below 50.degree. C. or water, as well as combinations of the two are preferably used. The water reacts with isocyanate, and in the process carbon dioxide is split off and polyurea is formed. The molded objects formed generally are closed celled. The density of the rigid foams lies between 5 and 1,000 kg/m.sup.3, preferably between 10 and 400 kg/m.sup.3 and especially between 20 and 60 kg/m.sup.3.
If rigid foams are produced with lower densities below about 23 kg/m.sup.3, a shrinkage generally occurs, that is, the cell structure, in the case of closed cells, can no longer withstand the external air pressure. This shrinkage is supported by the diffusion of the carbon dioxide through the intact cell membrane toward the surface of the foamed object, since a reduced pressure develops in the cells as a result.
If a dimensionally stable, very light polyurethane foam is to be produced, it is necessary, by suitable measures, to produce a foam with sufficient open cells. For this purpose, according to the state of the art, excess amount of water is used, for which no corresponding amount of isocyanate is available. Upon reaching the boiling point, the water evaporates and tears the cell membranes, unless the latter are stabilized excessively.
The so-called one-component foams are a special foam of rigid polyurethane foams. They are used especially for connecting or filling out workpieces, doors, door frames or hollow spaces in buildings or motor vehicles. For this purpose, the foam components are usually sprayed from a pressure container, such as an aerosol can, into the hollow spaces that are to be filled out, the reaction mixture being inflated by the expanding propellant gas. The resulting foam solidifies due to the reaction between the isocyanate groups and the moisture of the air into a dimensionally stable foam.
Such one-component polyurethane foam systems are described in the patent literature. Reference is made to the German patent 30 49 834 and to the German Offenlegungsschrift 38 29 104 as being representative of such literature.
An important condition for the industrial utilizability of the one-component foams obtained is the requirement of dimensional stability. The foam must not change its original shape when acted upon by the humidity of the air, by temperature, or by a change in air pressure. Since water vapor and carbon dioxide easily diffuse through the cell walls of the foam, the foam contracts when the surrounding atmosphere has a lower water vapor and/or carbon dioxide content than the cell spaces. Conversely, due to increased absorption of water vapor and/or lower carbon dioxide from the surrounding atmosphere, the foam can expand. In the same way, the foam can react to differences in atmospheric pressure.
There has been no lack of attempts to improve the dimensional stability of rigid polyurethane foams in general and of one-component foams in particular. This improvement is accomplished by opening, at least partially, the cells of the rigid polyurethane foam that is being formed.
The preparation of dimensionally stable rigid polyurethane foams is an object of the German patent 39 28 867. The objective is accomplished by the use of polysiloxane-polyoxyalkylene block copolymers, which have at least one aminofunctional group of the general formula ##STR1## linked to a silicon atom, wherein R.sup.4 is a divalent group,
R.sup.5 is a hydrogen or alkyl group with 1 to 4 carbon atoms, a polyether group having the formula --(C.sub.q H.sub.2q O).sub.2 R.sub.9 (q=2, 3 or 4; r=1 to 100; R.sup.9 =hydrogen or an alkyl group with 1 to 4 carbon atoms) or the group ##STR2## R.sup.6 is a divalent, aliphatic hydrocarbon group with 2 to 6 carbon atoms or a divalent aromatic hydrocarbon group, PA1 R.sup.7 and R.sup.8 are alkyl groups of 1 to 4 carbon atoms, which can have an OH group or be a common constituent of a 5- or a 6-membered ring, which may contain an oxygen or nitrogen atom, and PA1 Z has a numerical value of 0 or 1, PA1 Propylene oxide polyol, started with sucrose/glycerin and having an OH number of 520; PA1 Propylene oxide polyol, started with glycerin and having an OH number of 550; PA1 Preformulated polyol (contains ester polyols) with an isocyanate equivalent of 1.40; PA1 Polymeric diphenylmethane-4,4'-diisocyanate containing 31% isocyanate groups (crude MDI); PA1 Polysiloxane-polyether block copolymer (Tegostab.RTM. 8404 of Th. Goldschmidt AG); PA1 Polysiloxane-polyether block copolymer (Tegostab.RTM. 8450 of Th. Goldschmidt AG); PA1 Liquid polybutadiene with a molecular weight of 3,000 g/mole; PA1 Liquid polybutadiene with a molecular weight of 1,800 g/mole; and PA1 Liquid polyoctenylene with a molecular weight of 1,600 g/mole.
as a material for at least partially opening the cells of rigid polyurethane foams otherwise containing closed cells, in amounts of about 0.2 to 5% by weight, based on the prepolymer having polyol or isocyanate groups.
Aside from their cell-opening properties, said products also have a stabilizing action, so that they can be used not generally but only in narrow concentration ranges.
Improving the dimensional stability of the foams formed, by means of appropriate cell-opening additions, has also been attempted in the case of so-called one-component polyurethane foams. In this connection, reference is made particularly to the German patent 31 22 790. This patent discloses a method for the preparation of such one-component polyurethane foams. To increase dimensional stability of the cured foam during its preparation, 0.015 to 0.1% by weight of a dimethylpolysiloxane, with a dynamic viscosity of 20 to 300,000 mm.sup.2 /sec or a polysiloxane, containing perfluorinated alkyl groups and having a dynamic viscosity of 100 to 10,000 mm.sup.2 /sec, based on the polyesters and/or polyethers and the auxiliaries, are added to the mixture.
In actual fact, the foams, produced in accordance with the German patent 31 22 790, have an improved dimensional stability. However, it has turned out that, in order to achieve the desired effect, it is necessary to adhere to very narrow limits for the addition of the polysiloxanes. If the addition of polysiloxanes is too low, the desired effect does not occur or occurs incompletely. However, if the required amounts are exceeded, the polysiloxanes affect foam formation in an undesirable manner, so that there are foam disorders and, in limiting cases, the resulting foam even collapses. The so-called processing tolerance is undesirably narrow. These observations also apply quite generally for rigid polyurethane foams, which are prepared not by the special method of one-component foaming but, as described above, by conventional means.